The present invention generally relates to satellite communication systems. In particular, the present invention relates to optimizing communication over a satellite communications system by adjusting the boresight of an antenna on the satellite in response to system parameters.
A typical satellite communication system includes a satellite which communicates between various points on the earth's surface. Typically, a multibeam satellite communications system geographically divides the earth's surface into a number of circular or hexagonal geographic areas called cells. Each cell is serviced by different communication channels on the satellite. The communication channel between the satellite and the cell is typically referred to as a spot beam.
Because signals being transmitted at the same frequency interfere with one another, in a typical satellite communication system, spot beams in adjacent cells are operated at different frequencies. Thus, each spot beam is typically surrounded by a number of spot beams operating at different frequencies than the given spot beam. The geographic pattern of the frequencies of the spot beams is often referred to as a frequency re-use pattern. Typical frequency re-use patterns are 4 to 1 and 7 to 1 re-use patterns. In a 4 to 1 re-use pattern, for example, four different frequencies are employed to create the frequency re-use pattern.
Typically, a satellite communications system may produce several spot beams from a single satellite-mounted antenna. For example, the satellite-mounted antenna may be parabolic or spherical and multiple feeds may supply signals to a single antenna. The signals supplied by the multiple feeds may be directed to the desired cells using the geometry of the antenna. That is, the multiple feeds may be positioned to impinge on the antenna at different locations and/or incidence angles and thus be reflected to their desired cells. Thus, in this way, a single antenna structure may supply numerous spot beams.
Although a single antenna structure may supply several spot beams, each antenna has only a single boresight. The antenna's boresight is typically described as the “axis” of the antenna and is usually the location of greatest signal strength for the antenna. For example, in a spherically symmetric antenna, the boresight would be directed straight outward from the center of the antenna in the concave direction. Essentially, an antenna has only a single boresight because an antenna may only be mechanically oriented at one position at a single instance in time. The antenna's boresight is typically directed to the point on the earth's surface closest to the satellite, which is often called the sub-satellite point.
As mentioned above, signals being transmitted at the same frequency may interfere with one another. Although each spot beam is directed toward a single cell on the earth's surface, sidelobes of any spot beam may also occur. A sidelobe may be defined as the transmission of any power by the antenna in any direction other than the main, desired direction. For example, for any spot beam, the desired transmission direction is to its corresponding cell on the earth's surface. A sidelobe occurs where a fraction of the transmission power is not directed toward the desired cell and may fall anywhere on the earth's surface. The sidelobe may then interfere with communication in other cells. For example, a spot beam directed to cell A generates a sidelobe at a specific frequency that impinges on cell B. If cell B operates at the same frequency as cell A, then cell A's sidelobe interferes with operation in cell B. The interference may adversely affect the performance of the communication system and cause degraded communication performance, such as an increased bit error rate or a lower signal to noise ratio. The interference between two or more cells using the same frequency is often referred to as Co-Channel Interference (CCI).
The gain magnitude of sidelobes typically increases with angular deviation of the spot beam from the antenna's boresight. Thus, a spot beam directed to a cell at an angle of 7 degrees from the boresight of the antenna typically has a higher sidelobe level than a spot beam directed to a cell at an angle of 2 degrees from the boresight of the antenna. In other words, the strength of the sidelobes of spot beams scanned further from the antenna's electrical boresight is typically greater than the strength of the sidelobes of beams near the antenna's electrical boresight.
Additionally, sidelobe power typically diminishes with distance from the spot beam center. For example, take a system with three cells, cell A, cell B, and cell C, where the distance between cell A and cell B is less than the distance between cell A and cell C. If a spot beam is directed toward cell A and generates sidelobes, the sidelobes generally interfere with cell B more than cell C because cell B is closer to cell A.
Thus, for dense frequency re-use patterns, such as the 4 to 1 frequency re-use pattern mentioned above, because co-channel cells are spaced closely together, the CCI experienced by the cells may be particularly intense. That is, because cells utilizing the same frequency band are close together, the main lobe of each spot beam may be contaminated by the sidelobes of the surrounding spot beams utilizing the same frequency band. Conversely, in areas with a low density of antenna spot beams, interference generated by CCI decreases. This is because the spot beams utilizing the same frequency band are further apart, and the strength of the sidelobes decreases with distance.
Typically, in a satellite communication system frequency re-use plan, the geographic area representing North America is densely covered, often by using a closely-packed re-use plan such as the 4 to 1 frequency re-use plan. Conversely, South American coverage is typically far less dense with most systems only providing coverage on the coasts or at various population centers.
As mentioned above, the satellite's antenna is typically boresighted at a sub-satellite point. Typically the sub-satellite point is on the earth's surface nearest the satellite. Alternatively, the boresight of the antenna may be positioned so that the angular deviation from the boresight of the most distant cell in the frequency re-use pattern is minimized. For example, in a communications system that provides services to both North America and South America, the antenna may be boresighted so that the boresight lies midway between the northernmost cell (Alaska, for example) and the southern most cell (Argentina, for example). Recall that decreasing the angle between boresight and spot beam serves to minimize sidelobe generation, and thus CCI. Consequently, minimizing the maximal angular deviation between boresight and spot beam for the whole frequency re-use pattern serves to minimize the sidelobe generation and CCI for the whole system.
Any minimization of CCI results in an improvement in overall system performance, for example, improved noise floor or improved Bit Error Rate (BER). Consequently, any improvement in CCI is intensely commercially desirable.
Thus, a need has long been felt for a system and method for providing improved CCI for a satellite communication system. A need has especially been felt for such a system that improves CCI, thus providing improved system performance, such as improved noise floor or BER, for example.